The present invention relates to a system for manufacturing a very small flow orifice in which the shape, size, and surface finish of the orifice significantly affect the character of the flow through the orifice. In one particular embodiment, the invention permits a very high degree of control over manufacture of an orifice of a fuel injector so that the injector exhibits repeatable flow characteristics.
An electro discharge machine (EDM) can be used to produce very small flow orifices, i.e., on the order of less than one millimeter in diameter. The operating parameters of the EDM affect the character of the orifices so produced. Some of these parameters are controlled by the operator. For example, changes to the feed rate of the spark erosion electrode and the peak current intensity supplied to the electrode, or changes to the size in the electrode diameter, each affect the orifice produced by the EDM. Other factors include on and off times, servo and power settings.
In addition, there are a number of factors which are not controlled by the operator yet nonetheless affect the flow characteristics of the small orifice produced by an electro discharge machine. Some of the more significant factors relate to the EDM. For example, the electrode burns away during use and often begins to taper and/or get out of round as it burns away. Variation also occurs in electrode diameter, both between electrodes of different nominal sizes and between electrodes of the same nominal size. The electrode also can be damaged by contamination. Moreover, the processing of the parts presented to the EDM can vary. For example, parts from different processing batches (such as a turning or a stamping process) can be presented with thickness variations, or the heat treatment layer thicknesses of parts can vary among parts within the same processing batch.
As noted above, fuel injectors have one or more flow orifices, which are examples of a kind of small orifice.
Several methods are known for testing different aspects of fuel injectors. Air or nitrogen has been applied to the fully assembled injector at a known pressure so that a gauge can be used to detect any reduction in that pressure due to leaks. Air has also been used to check the flow characteristics of a fully assembled fuel injector or just an exiting orifice plate of a fuel injector. However, this air flow test method has proven inaccurate due to the low level of repeatability resulting from moisture and contaminants in the air as well as variations experienced due to the temperature of the supplied air. Moreover, moisture in the test air can cause corrosion of a flow orifice, and contaminants in the test air also can damage an orifice and change its flow characteristics.
In addition, the flow characteristics of a fuel injector or just an orifice plate can be tested using the method of flowing a liquid known as Stoddard solvent through a fully assembled injector or an orifice plate assembled in a mock injector assembly and measuring whether the flow rate is acceptable. In a mock assembly, the orifice plate is clamped into the injector. In a true assembly, the orifice plate is welded into the injector.
As known in the art, Stoddard solvent mimics the flow rate characteristics of gasoline. Stoddard solvent is not as flammable as gasoline and thus is more suitable for tests conducted in a manufacturing environment.
Stoddard testing or air testing a completed injector has its drawbacks. In both cases, the injector is in an assembled state before flow testing can be conducted. Time delays resulting from the time needed to complete the testing can result in the fabrication of a large number of failed injectors before the condition of an improper orifice can be detected in a finished product. Stoddard testing an orifice plate in particular requires sophisticated flow equipment, is very time consuming, and the test results correlate poorly to the finished products.
In order to produce a fuel injector orifice plate with an orifice sized according to a desired flow specification, the EDM operator selects set up conditions (i.e., electrode diameter size, guide size, etc.), and an initial operating condition for the EDM based upon prior experience in producing that particular orifice. The orifice is made with the EDM set at the selected operating conditions. To provide information feedback for controlling the orifice manufacturing process, the orifice plate would be subjected to a Stoddard test in a mock injector assembly. This test requires about ten minutes or so to perform. Moreover, a very sophisticated and expensive piece of equipment is required to perform the Stoddard test, and this equipment occupies a significant amount of space on the factory floor. Depending upon the result of this Stoddard test, the operator again adjusts the EDM operating parameters according to the judgment and experience of the operator. A new orifice is fabricated and subjected to the Stoddard test.
This continues until several orifice plates with flow that is deemed acceptable are obtained in succession. Then production begins.
Since it is prohibitively uneconomical to provide a single Stoddard machine for each EDM, the availability of the Stoddard machine determines how many orifice plates are tested. Normally, only about 10% of the orifice plates are tested. Depending upon the number of EDM's being serviced by a particular Stoddard test machine, thirty or forty minutes may elapse between testing of an orifice produced by a particular EDM. If the tested orifice proves faulty, then a substantial number of defective orifice plates would have been produced by that EDM. Moreover, because of the long time needed to perform the present test method, the number of manufactured parts being tested (the test sample) is very small relative to the total number being manufactured. This fact prevents the operator from obtaining an accurate indication of where in the realm of the flow specification, the parts being made are distributed. This results from the lack of statistical confidence in low sample sizes. Thus, correcting for the degree to which the orifice is unacceptable is largely a matter of the individual operator's judgment based on experience. Often, the operator changes the operating parameters of the EDM more than is required.
In FIG. 1, the solid line curve shows a possible distribution of parts relative to the part specification illustrated by the two vertical lines. The value "k" in FIG. 1 represents a specification parameter (e.g., a Stoddard flow value) near the lower limit. Because of the small number of parts that can be tested, the operator is ignorant of the shape or relative location of the distribution curve shown in FIG. 1. Thus, even though the operator knows the coordinates for the part, the operator does not know where the distribution curve falls relative to this part's coordinates. Thus, when the EDM operator tests a part at value "k" of the specification parameter, the operator may decide to adjust the EDM operating conditions to produce parts nearer to the higher specification parameter limit. This adjustment could, depending upon the actual position of the distribution relative to the tested part, produce a frequency distribution of additional parts like the dashed line. As shown in FIG. 2, which combines the curves of FIG. 1, this unneeded adjustment results in a wider overall distribution of orifice flow rates being produced and more parts produced near the opposite limit of the parts specification. Accordingly, more unacceptable orifices are produced.
It also is known to combine several tests, such as Stoddard tests and air tests in sequence in an attempt to cull the unacceptable parts and eliminate testing errors. However, 100% testing to obtain zero defects of parts produced is not economical because of the costs of the increased time devoted to testing and the greater number of test equipment units which must be employed.